The present invention relates to a mechanical seal in connection with a shaft seal, and more specifically to a mechanical seal which is used for a shaft seal portion of a rotating device such as a pump for pumping liquids containing a large quantity of slurries.
One example of the aforesaid pump has been known in which as shown in FIG. 3, an impeller 2 is mounted or the end of a shaft 1 extending from a drive source such as a motor to pump a liquid of slurry containing a large quantity of bauxite or coal powder by rotational operation of the impeller 2. A mechanical seal 11 functioning as a shaft seal device is provided at a boundary between a space 3 on the impeller 2 side and a space 4 on the motor side so as to prevent the slurry liquid from flowing toward the motor side.
A mechanical seal generally designated at 11 has a non-rotational seal ring 12 airtightly carried on the side of a pump housing 5 and a rotational seal ring 13 carried on the shaft 1 side. Both the seal rings 12 and 13 are axially opposed to present slurry liquid S present on the outside diameter of the seal rings 12 and 13 from passing by the opposed rings to the inside diameter.
The seal ring 13 on the rotational side is embraced by a seal cover 15 through a rubber spring 14, the seal cover 15 being fitted in the outer periphery of a sleeve 16 slipped over the shaft 1, and the seal cover 15 and the sleeve 16 are secured to the sleeve 16 or the shaft 1 by means of set screws 17 and 18, respectively. The seal ring 13 on the rotational side is rotated following the shaft 1 together with the sleeve 16, the seal cover 15 and the rubber spring 14.
The seal ring 12 on the non-rotational side is mounted through a rubber spring 21 to flanges 19 and 20 bolted to the pump housing 5. Reference numeral 22 denotes a set plate detachably bolted to the outer end of the flange 20, said set plate having its internal diameter end engaged with a groove of the sleeve 16. In collectively mounting on the pump various members on the stationary side (non-rotational side) composed of the flanges 19, 2O, the rubber spring 21 and the seal ring 12 and various members on the rotational side composed of the sleeve 16, the seal cover 15, the rubber spring 14 and the seal ring 13, the aforesaid set plate 22 is used to locate both member groups one another. When the pump is used, the set plate 22 is removed.
In the pump provided with the aforesaid mechanical seal, the slurry liquid pumped by the rotational operation of the impeller 2 is taken from the intake side shown at A and transferred toward discharge side likewise shown at B. As viewed as a general flow passage, the mechanical seal 11 is to be positioned at the dead end portion behind the impeller 2. Accordingly, when part of the slurry liquid flows into the dead end portion as indicated by the arrow in FIG. 3, the slurry contained in the slurry liquid no longer has nowhere to go, and as the result the slurry is accumulated in the neighbourhood of the seal portion of the mechanical seal, resulting in inconveniences in that part of the slurry is forced into the seal portion to cause wear of the important opposed portion of the seal rings 12 and 13 and the like.
To cope with the aforementioned accumulation of slurry, preventive measures in the form of self-flushing have been normally taken. However, in the case where the viscosity of the slurry is high, the flushing hole is sometimes clogged by the slurry. Alternatively, it has been considered to introduce external water into the accumulated portion for flushing. However, in the case where the external water cannot be introduced into the slurry liquid (in the case where they cannot be mixed), the aforesaid method cannot be employed. Accordingly, in these cases, the accumulation of slurry still occurs in the neighbourhood of the seal portion, thus resulting in inconveniences in that the slurry liquid leaks out due to the wear of the seal rings 12 and 13, such that the seal rings 12 and 13 have to be replaced early. It is an object of the present invention to prevent the accumulation of slurry.